Use of aqueous solutions of polymers such as a partially hydrolyzed polyacrylamide (PHPA) to recover residual oil from oil-bearing subterranean formations is well known. In secondary recovery operations, after normal drilling and pumping operations, the subterranean formation is flooded through an input well with a polymer solution and the resulting admixture of polymer solution and oil is forced to an output well head where it is pumped from the ground. In tertiary operations, recovery of residual oil is first accomplished by flooding the rock formation with water and, thereafter, flooding with a polymer solution.
Through extensive research in the use of polymer solutions in secondary and tertiary oil recovery operations, it has been discovered that a polymer solution can be tailor-made, so to speak, to meet the performance demands of substantially any oil-bearing subterranean formation. More specifically in this connection, it has been found that such considerations as the average molecular weight and the molecular weight distribution properties of a polymer comprising the polymer solution can significantly augment and enhance oil recovery thereby resulting in important reductions in recovery costs.
In the case of solutions of PHPA, care must be taken in formulating, diluting and handling the solutions in order to limit the breaking up or "degrading" of the polymer and thus to preserve to the greatest extent possible its preselected average molecular weight and molecular weight distribution properties. The on-site and on-demand production and use of PHPA accentuates the problem of polymer degradation.
Apparatus for the production and dilution of PHPA may be maintained on a continuous basis. The direction and control of flow of the aqueous PHPA as it is transported to the point of injection into an oil-bearing formation are critical in maintaining the integrity of the polymer. Mechanical stress, such as that induced by abrupt changes in flow direction, turbulent flow, and travel through partially closed valves or other flow control devices contribute to the degradation of the polymer solution, and adversely affect such properties as the polymer's mobility, injectivity, brine tolerance, and resistance to further thinning induced by shear forces.
The systems presently used to prepare and inject aqueous PHPA solutions for oil recovery purposes can be preassembled and mounted on skids, for example, for ready transport to and from a site where recovery is to take place. Such a system may include a monomer supply, a source of water, polymerization apparatus, catalyst feed and monitoring equipment, hydrolyzation apparatus including means for feeding a controlled amount of a hydrolyzing agent into the polymer stream, and apparatus for diluting the hydrolyzed polymer and injecting it into an input well penetrating a reservoir of interest.
A system of this type is capable of producing a broad spectrum of polymers of varying average molecular weight and molecular weight distribution to meet the permeability demands of substantially any oil-bearing formation being worked. Once the parameters are determined, the system can produce a polymer having the desired properties. The present invention is intended to maintain the preselected characteristics of the polymer solution as the solution is moved through a system to the input well, while enabling the rate of solution flow to be properly controlled.
In the past, flow control expedients have included varying the length of pipe through which the polymer solution is transported or the use of sand packs. These techniques are unsatisfactory because they are cumbersome, and require much time and labor to effect flow changes.
Other prior art efforts involving the manufacture and transport of polymer solutions have not addressed the particular problems solved by the present invention.
U.S. Pat. No. 3,034,526, for example, describes a three-dimensional T-shaped cascade system intended to prevent the degrading of molten, highly viscous linear polymers such as nylon, but does not teach a simple and efficacious manner to vary polymer flow rates while avoiding degradation.
U.S. Pat. No. 3,128,794 describes apparatus for moving molten polymers through pipe lines and using inverters positioned in the flow path of the polymer to equalize residence time between the polymer flow segment at the outer periphery of the pipe line and the flow segment at the center of the pipe line. The patent merely teaches the diversion of polymer flowing along the conduit walls toward the conduit center, and vice versa, without controlling the rate of flow. A similar consideration, pipeline residence time, is addressed in U.S. Pat. No. 3,353,564, which uses a plurality of screens spaced apart one from the other and placed in the material flow path. The positioning of such screens is intended to prevent thermal degradation of the polymer by flattening the velocity profile of the polymer to equalize flow rates rather than to set flow rates.
U.S. Pat. No. 3,945,402 teaches a pipe system incorporating turbulence control apparatus which includes spaced-apart screens positioned within a tapered pipe run. The internal wall roughness of the '402 apparatus is selected to achieve laminar flow at Reynolds numbers in excess of 2200. Although use of valves and pumps in the system is described, no teaching is found concerning use of a system to control the transport of mechanically degradable polymer solutions.